1. Field of Invention
The invention relates to a magnetic-inductive flowmeter for measuring the rate of flow of a flowing medium, with a measurement line, a magnetic field generating means for generating a magnetic field which at least partially penetrates the measurement line, and with two measurement electrodes for detecting and tapping a measurement voltage which has been induced in the flowing medium, the measurement line being provided at least partially with an electrically insulating cover layer inside, and the measurement voltage can be galvanically or capacitively tapped by the measurement electrodes. Here, a magnetic-inductive flowmeter is assumed whose measurement line is electrically conductive and which, therefore, has a metallic measurement line; only then is at least partially an electrically insulating cover layer necessary. This electrically insulating cover layer is provided at least in the region of the measurement electrodes. The dimension which the cover layer should at least have depends on the diameter of the measurement line. Usually the cover layer in the lengthwise direction of the measurement line has expansion which corresponds to roughly at least twice the diameter of the measurement line. However, the measurement line can, then, also partially or entirely be provided with a cover layer when it is not electrically conductive. Usually, but not functionally necessarily, does a housing which holds the measurement line, and preferably also the magnetic field generating means and the measurement electrodes, belong to the magnetic-inductive flowmeter of the type under consideration, the housing conventionally having a round cross section.
2. Description of Related Art
Magnetic-inductive flowmeters have been very widely known for decades in the prior art; in this regard, reference is made by way of example to literature citation Technical Flow Rate Measurement by Prof. Dr. Ing. K. W. Bonfig, 3rd edition, Vulkan-Verlag Essen, 2002, pages 123 to 167 (compare also, German patent applications 196 37 761, 197 52 368 and 10 2007 003 614 and European patent applications 0 274 768 (U.S. Pat. No. 4,920,795), 0 762 084 (U.S. Pat. No. 5,925,830) and 1 764 587 (U.S. Pat. No. 7,412,901)).
The underlying principle of a magnetic-inductive flowmeter for measurement of the rate of flow of a flowing medium goes back to Faraday who as early as 1832 suggested using the principle of electrodynamic induction for measurement of the flow velocity of a flowing medium. According to the Faraday Induction Law, in a flowing medium which is entraining charge carriers and flowing through a magnetic field, an electrical field intensity forms perpendicular to the flow direction and perpendicular to the magnetic field. The Faraday Induction Law is used in a magnetic-inductive flowmeter by a magnetic field being generated by a magnetic field generating means which conventionally has two energized magnet coils and being routed at least partially through a measurement line, the generated magnetic field having at least one component which runs perpendicular to the flow direction. Within the magnetic field each volumetric element of the flowing medium which has a certain number of charge carriers and which is moving through the magnetic field with the field intensity which arises in this volumetric element contributes to a measurement voltage which can be tapped galvanically or capacitively and which can be detected by the measurement electrode.
If, as stated above, magnetic-inductive flowmeters have been extensively known for decades in an almost unimaginable number of embodiments, with reference to magnetic-inductive flowmeters, as in many extensively developed areas of engineering, progress is still desirable and also possible. This also applies to a special degree to the measurement electrodes of these magnetic-inductive flowmeters.
In magnetic-inductive flowmeters of the type under consideration, the flowing medium can be a gas-free or at least largely gas-free liquid, but the flowing medium can also be a liquid which contains more or fewer portions of gas. Otherwise in the magnetic-inductive flowmeters under consideration the measurement line can be completely filled or also only partially filled according to operation.
It was stated at the beginning that the invention relates to a magnetic-inductive flowmeter for measuring the flow rate of a flowing medium. This magnetic-inductive flowmeter however allows not only measurement of the rate of flow of a flowing medium, but also measurement of conductivity and/or measurement of the liquid level, with liquid level measurement the information being available whether the measurement line is completely or partially filled. Consequently a magnetic-inductive flowmeter of the initially explained type can be used for flow rate measurement, for conductivity measurement and/or for liquid level measurement. Conventionally, magnetic-inductive flowmeters are used not only for conductivity measurement and not only for liquid level measurement. If rather there is also conductivity measurement and/or liquid level measurement, a magnetic-inductive flowmeter of the type under consideration is generally used for measurement of flow rate and measurement of conductivity, for flow rate measurement and liquid level measurement or for flow rate measurement, conductivity measurement and liquid level measurement. But it is also possible to use this magnetic-inductive flowmeter also only for conductivity measurement, only for liquid level measurement or only for conductivity measurement and liquid level measurement. But a magnetic-inductive flowmeter for measuring the flow rate of a flowing medium is always assumed below. Nevertheless, what is detailed below also always applies to a magnetic-inductive flowmeter for flow rate measurement and for conductivity measurement, for flow rate measurement and for liquid level measurement, for flow rate measurement, for conductivity measurement and for liquid level measurement, for conductivity measurement, for liquid level measurement or for conductivity measurement and liquid level measurement.
Magnetic-inductive flowmeters in which the measurement line at least partially has an electrically insulating cover layer are numerously known (compare German patent applications 10 2004 057 696 (U.S. Pat. No. 7,272,978), 10 2006 026 310, and 10 2006 026 311, German patent 43 27 826 and European patent application 0 608 793). The material for the cover layer which can have a layer thickness in the range from 0.1 to 500 microns, optionally even more, are generally plastics, for example polyurethane, polyolefin, polytetrafluorethylene or polyether ether ketone. Otherwise in magnetic-inductive flowmeters, providing between the measurement line and the cover layer, often also called a liner, a primer which imparts adhesion of the cover layer to the measurement line is also known, and the primer can also be plastic of the same or another type.